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Strain Measuring.

381

shackle £, a very good axial thrust is obtained. Professor Unwinds
shackles at BX receive the test piece between a hard block <?, and
spherical surfaces d, and the parts are shewn separately to make
their construction clear. The Emery machine is provided, for
compression,- with spherical nuts A and B, upon which lie convex
plates or tables D and c, and the hard seatings E F receive the
thrust, c and D are adjusted to the specimen by means of the
handles jj. In the shearing shackles at Fig. 339 (designed by
Mr. Wicksteed for Professor Hele-Shaw), a knife A adjusts itself
so as to give equal pressure at B and c, while the specimen is
nicked down to localise the strain. The torsion grips at A, Fig. 336,
have sockets to receive a square bar turned down in the mid portion;
and Fig. 340 illustrates a pair of bending shackles where knife
edges B B are adjustable for various lengths of specimen, and the
shackle A is formed so as to indent the bar as little as possible.

Strain Measuring.—At first it was considered sufficient to
know the breaking load in tension, then Mr. Hodgkinson shewed
the necessity for compression tests, and Mr. Kirkaldy lastly
pointed out that the contraction of area at fracture was not to be
overlooked. Now it is considered imperative to know the
breaking load and elongation (usually given per cent., or extension
x 100), and advisable to obtain both load and extension within
the elastic limit. A stress-strain diagram, as in Fig. 321, will shev*
the whole life of the bar, and can be obtained in two ways: (i)
by noting load and extension at several points during the experi-
ment (the latter being measured by instruments of more or less
precision), then plotting a diagram to these dimensions; or (2)
by compelling the machine to make an autographic diagram.

Taking (i), the simplest method is to make a centre pop near
each end of the specimen, and measure the distance between
these by means of dividers; a better result is obtained by the
use of a standard rod c (Fig. 341), and wedge gauge D, placed
between clamps A and B on the specimen; and very great accuracy
by the aid of an exte&someter. Such an instrument is absolutely
necessary for the fine extensions within the elastic limit, and Fig.
342 shews a verf effective form devised by Professor Unwin. A
is the specimen to which Tee brackets c and D are clamped, both
of which carry spirit levels F and j, while D in addition supports
the measuring pillar G. Within G is a fine screw carrying a